1. Field of the Invention
The present invention relates to a fuel injection system of an internal combustion engine capable of improving accuracy of fuel injection.
2. Description of Related Art
A fuel injection system of an internal combustion engine mounted on a vehicle and the like supplies fuel into cylinders of the engine through injectors. The fuel injection system includes a control device for setting injection conditions such as an injection quantity or injection timing in accordance with a pressed degree of an accelerator as required torque or sensed operating states. Errors are caused in an actual injection quantity or actual injection timing due to operating characteristics of the injector and the like. Correction of the errors is important for improving accuracy of the fuel injection. A technology disclosed in Unexamined Japanese Patent Application Publication No. 2002-250247 (Patent Document 1) stores a weighting characteristic curve, in which a change in the injector with time is reflected, and calculates the change in the injector with time in reference to the weighting characteristic curve. Thus, the technology sets a correction value for correcting a driving period of the injector. An exponential function is proposed as an example of the weighting characteristic curve.
However, since the operating characteristics of the injector are not simple, the injection accuracy cannot be improved by simply reflecting the correction value in the driving period by multiplication or addition. Characteristics of the injection quantity Q with respect to the driving period TE are shown in FIG. 4. As shown in FIG. 4, generally, the injection quantity Q gradually increases as the driving period TE increases. A gradient of the increase of the injection quantity Q varies in accordance with an injection pressure Pc. The injection pressure Pc increases along a direction of an arrow mark Pc in FIG. 4. The characteristics before the change in the injector with time are shown by solid lines a, b, and c in FIG. 4, and the characteristics after the change in the injector with time are shown by broken lines a′, b′ and c′ in FIG. 4. In a certain area (a point A in FIG. 4, for instance) of the driving period TE and the injection pressure Pc, the change in the injector with time affects the injection quantity Q so that the injection quantity Q increases. In another area (a point B in FIG. 4, for instance) of the driving period TE and the injection pressure Pc, the change in the injector with time affects the injection quantity Q so that the injection quantity Q decreases. Therefore, even if the single weighting characteristic curve is used as in the technology of Patent Document 1, the error in the fuel injection due to the change in the injector and the like with time cannot be absorbed (corrected) sufficiently. Since the characteristics of the injection quantity are not simple, the correction value cannot be suitably provided by a simple function such as an exponential function. Thus, the technology disclosed in Patent Document 1 cannot improve the injection accuracy sufficiently.